The Internet provides users with convenient and ubiquitous access to digital content. Because of the potential of the Internet as a powerful distribution channel, many CE products strive to directly access the Internet or to interoperate with the PC platform—the predominant portal to the Internet. The CE products include, but are not limited to, digital set top boxes, digital TVs, game consoles, PCs and, increasingly, hand-held devices such as PDAs, mobile phones, and mobile storage and rendering devices, such as Apple's iPod. The use of the Internet as a distribution medium for copyrighted content creates the compelling challenge to secure the interests of the content provider. In particular it is required to warrant the copyrights and business models of the content providers. Increasingly, CE platforms are operated using a processor loaded with suitable software. Such software may include the main part of functionality for rendering (playback) of digital content, such as audio and/or video. Control of the playback software is one way to enforce the interests of the content owner including the terms and conditions under which the content may be used. Where traditionally many CE platforms (with the exception of a PC and PDA) used to be closed, nowadays more and more platforms at least partially are open. In particular for the PC platform, some users may be assumed to have complete control over the hardware and software that provides access to the content and a large amount of time and resources to attack and bypass any content protection mechanisms. As a consequence, content providers must deliver content to legitimate users across a hostile network to a community where not all users can be trusted. The general approach in digital rights management for protected content distributed to CE platforms is to encrypt the digital content (for instance using DES or AES) and to store the decryption key (or the “license”) in a so-called License database (for a PC, the License database is typically stored on the PC's hard disk). The largest vulnerability of digital rights management relying on encryption is the key distribution and handling. For playback, a software player has to retrieve a decryption key from the license database, it then has to store this decryption key somewhere in memory for the decryption of the encrypted content. This leaves an attacker two options for an attack of the key handling in a software player: firstly, reverse engineering of the license database access function could result in a black box software (i.e., the attacker does not have to understand the internal workings of the software function) capable of retrieving asset keys from all license databases. Secondly, by observation of the accesses to memory used during content decryption it is possible to retrieve the asset key.
Typically, digital rights management systems use an encryption technique based on block ciphers that process the data stream in blocks using a sequence of encryption/decryption steps, referred to as rounds. During each round, a round-specific function is performed. The round-specific function may be based on a same round function that is executed under control of a round-specific sub-key. For many encryption systems, the round function is specified using mapping tables for different parts of the domain of the function. Even if no explicit tables were used, nevertheless frequently tables are used for different parts of the domain of the function for efficient execution in software of encryption/decryption functions. The computer code accesses or combines table values into the range value of the function. Instead of distributing keys, that may be user-specific, it becomes more interesting to distribute user specific algorithms instead of keys for en- or decryption algorithms. These algorithms, most often functions (mappings), have to be obfuscated (hidden) in order to prevent redesign or prohibit the re-computation of elements that are key-like. On computers, tables accompanied with some computer code often represent these functions.